Graptolite

Certain fossils from the Ordovician and Silurian Periods
of the Paleozoic Era resemble saw blades sketched
on the rock in pencil. As it turns out, these fossils appear all
over the world. Not only that, certain types of the fossils appear only during well-defined time periods, and are invaluable tools for dating and organizing rock strata from those periods. They are so important, that an entire lexicon of terms has been invented to describe their morphological characteristics.

These peculiar fossils are the remains of graptolites, colonial
animals that either floated as macroplankton in the ocean or lived benthically,
attatched to the sea bottom. Each colony, or "rhabdosome", was
composed of one or more long bodies, or "stipe"s. The stipes
of planktonic species were arranged radially; the stipes of benthic graptolites
had a dendritic habit. The individual organisms, or "zooids", lived
in little cups or "thecae" strung out along the stipes. Each zooid
was a clone (produced, presumably, by budding) of a nearby zooid, and
presumably all of the zooids were descended from a single original organism,
or "sicula" (presumably produced sexually) which lived in a theca
at the root of the rhabdosome.

A planktonic rhabdosome's most important function was to keep its members
floating by the plankton they fed on, and various species evolved different
techniques to keep from settling to the sea bottom, and death. Although
some appear to have grown gas-filled bladders attatched at the center of
a wheel of stipes, the most common adaptation was to reduce the number
of stipes, so that by the mid-Silurian, single-stipe rhabdosomes that floated
as a big conical coil had evolved.

421 million years ago, at the end of the Wenlock epoch of the Silurian,
a period of rapid global cooling followed by rapid global warming caused
a mass extinction that killed off 95% of all graptolite species.
A few dendroid forms lived on until the Mississippian Period. Although
graptolites had no hard parts to fossilize, if one fell into the right
type of anoxic mud after death, its remains would turn into a thin carbonized
layer in the shale formed from that mud. Stipes would resemble
saw blades with the thecae as the "teeth". Just the right thing for
a paleontologist to dig up and ponder several hundred million
years later.

The graptolites' closest modern relatives appear to be the Pterobranchs, which closely resemble dendroid graptolites. Not only that, British pterobranch expert Peter N. Dilly recently showed that pterobranchs exhibit a type of exoskeleton found elsewhere only on graptolites, and claimed that pterobranchs were "living graptolites".

Another group, the Enteropneusta or acorn worms, has been associated with graptolites. Originally, the acorn worm's dorsal nerve chord
made scientists think that it was a primitive ancestor of vertebrates,
and so they and the graptolites were placed relatively close to the Urochordata (tunicates) and Chordata(vertebrates). However, it was later shown that acorn worms are more closely related to echinoderms than chordates, and so they, the pterobranchs, and the graptolites were moved into their
own phylum, Hemichordata.

In 2001, Claus Nielsen of the University of Copenhagen's Zoological Museum claimed that acorn worms really aren't related to the graptolites and pterobranchs at all. Nielsen separates the acorn worms into their own phylum Enteropneusta and elevates to another phylum, Pterobranchia.

One of numerous species of slender and delicate fossils, of the genus Graptolites and allied genera, found in the Silurian rocks. They belong to an extinct group (Graptolithina) supposed to be hydroids.